Modern societies are critically dependent on energy to maintain their standards of living and economic viabilities. All aspects of modern life, ranging from the generation of electricity to the powering of automobiles, require the consumption of energy. Conventional fossil fuels are primarily used to meet the energy needs of today's societies. As more societies modernize and existing modern societies expand, the consumption of energy continues to increase at ever growing rates. The increased worldwide use of fossil fuels is creating a number of problems. First, fossil fuels are a finite resource and concern is growing that fossil fuels will become fully depleted in the foreseeable future. Scarcity raises the possibility that escalating costs could destabilize economies as well as the likelihood that nations will go to war over the remaining reserves. Second, fossil fuels are highly polluting. The greater combustion of fossil fuels has prompted recognition of global warming and the dangers it poses to the stability of the earth's ecosystem. In addition to greenhouse gases, the combustion of fossil fuels produces soot and other pollutants that are injurious to humans and animals. In order to prevent the increasingly deleterious effects of fossil fuels, new energy sources are needed.
The desired attributes of a new fuel or energy source include low cost, plentiful supply, renewability, safety, and environmental compatibility. Hydrogen is currently the best prospect for these desired attributes and offers the potential to greatly reduce our dependence on conventional fossil fuels. Hydrogen is the most ubiquitous element in the universe and, if realized, offers an inexhaustible fuel source to meet the increasing energy demands of the world. Hydrogen is available from a variety of sources including coal, natural gas, hydrocarbons in general, organic materials, inorganic hydrides and water. These sources are geographically well distributed around the world and accessible to most of the world's population without the need to import. In addition to being plentiful and widely available, hydrogen is also a clean fuel source. Combustion of hydrogen produces water as a by-product. Utilization of hydrogen as a fuel source thus avoids the unwanted generation of the carbon and nitrogen based greenhouse gases that are responsible for global warming as well as the unwanted production of soot and other carbon based pollutants in industrial manufacturing. Hydrogen truly is a green energy source.
The realization of hydrogen as a ubiquitous source of energy ultimately depends on its economic feasibility. Economically viable methods for producing hydrogen as well as efficient means for storing, transferring, and consuming hydrogen, are needed. Chemical and electrochemical methods have been proposed for the production of hydrogen. The most readily available chemical feedstocks for hydrogen are organic compounds, primarily hydrocarbons and oxygenated hydrocarbons. Common methods for obtaining hydrogen from hydrocarbons and oxygenated hydrocarbons are dehydrogenation reactions and oxidation reactions.
Steam reformation and the electrochemical generation of hydrogen from water through electrolysis are two common strategies currently used for producing hydrogen. Both strategies, however, suffer from drawbacks that limit their practical application and/or cost effectiveness. Steam reformation reactions are endothermic at room temperature and therefore require heating. Temperatures of several hundred degrees are generally needed to realize acceptable reaction rates. These temperatures are costly to provide, impose special requirements on the materials used to construct the reactors, and limit the range of applications. Steam reformation reactions also occur in the gas phase, which means that hydrogen must be recovered from a mixture of gases through a separation process that adds cost and complexity to the reformation process. Steam reformation also leads to the production of the undesirable greenhouse gases CO2 and/or CO as by-products. Water electrolysis has not been widely used in practice because high expenditures of electrical energy are required to effect water electrolysis. The water electrolysis reaction requires a high minimum voltage to initiate and an even higher voltage to achieve practical rates of hydrogen production. The high voltage leads to high electrical energy costs for the water electrolysis reaction and has inhibited its widespread use.
In U.S. Pat. No. 6,607,707 (the '707 patent), the disclosure of which is incorporated by reference herein, the instant inventors considered the production of hydrogen from hydrocarbons and oxygenated hydrocarbons through reactions of hydrocarbons and oxygenated hydrocarbons with a base. Using a thermodynamic analysis, the instant inventors determined that reactions of many hydrocarbons and oxygenated hydrocarbons react spontaneously with a base or basic aqueous solution to form hydrogen gas at particular reaction conditions, while the same hydrocarbons and oxygenated hydrocarbons react non-spontaneously in conventional steam reformation processes at the same reaction conditions. Inclusion of a base was thus shown to facilitate the formation of hydrogen from many hydrocarbons and oxygenated hydrocarbons and enabled the production of hydrogen at less extreme conditions than those normally encountered in steam reformation reactions, thereby improving the cost effectiveness of producing hydrogen gas. In many reactions, the processes of the '707 patent led to the formation of hydrogen gas from a liquid phase reaction mixture, in some cases at room temperature, where hydrogen was the only gaseous product and thus was readily recoverable without the need for a gas phase separation step. The reactions of the '707 patent further operate through the formation of carbonate ion or bicarbonate ion and avoid the production of the greenhouse gases CO and CO2. Inclusion of a base creates a new reaction pathway for the formation of hydrogen gas with thermodynamic benefits that allow for the production of hydrogen gas at lower temperatures than are needed for corresponding steam reformation processes.
In co-pending U.S. patent application Ser. No. 10/321,935 (the '935 application), the instant inventors considered electrochemical methods to promote the production of hydrogen from organic substances in the presence of water (or acidic solution) and/or a base. They showed that electrochemical reactions of organic substances with water to produce hydrogen require lower electrochemical cell voltages than water electrolysis. They also showed that electrochemical reactions of organic substances in the presence of an acid or base require low electrochemical cell voltages at room temperature. In some embodiments, hydrogen production reactions of organic substances were shown to occur at room temperature in an electrochemical reaction and were accelerated by heating. In other embodiments, hydrogen production reactions of organic substances were shown to occur spontaneously at room temperature without applying a voltage and were accelerated by providing a voltage.
In co-pending U.S. patent application Ser. No. 10/636,093 (the '093 application), the instant inventors recognized that the realization of the beneficial properties of the reactions described in the '707 patent and the co-pending '935 application requires a system level consideration of the costs and overall efficiency of the reactions. In addition to energy inputs and raw materials, consideration of the disposal or utilization of by-products must be made. Of particular importance is consideration of the dispensation of the carbonate and bicarbonate ion products of the disclosed hydrogen producing reactions. In the co-pending '093 application, the instant inventors describe strategies for the recycling of the carbonate and bicarbonate ions. A carbonate recycle process was described that includes a first step in which carbonate ion is reacted with a metal hydroxide to form a soluble metal hydroxide and a weakly soluble or insoluble carbonate salt. The soluble metal hydroxide may be returned to the hydrogen producing reaction as a base reactant for further production of hydrogen. In a second step, the carbonate salt is thermally decomposed to produce a metal oxide and carbon dioxide. In a third step, the metal oxide is reacted with water to reform the metal hydroxide used in the first step. The carbonate recycle process is thus sustainable with respect to the metal hydroxide and the overall hydrogen producing process is sustainable with respect to the base through the carbonate recycling process of the '093 application. Bicarbonate by-products of hydrogen producing reactions of organic substances with bases can be similarly recycled according to the '093 application by first converting a bicarbonate by-product to a carbonate and then recycling the carbonate.
The hydrogen producing reactions of the '707 patent and '935 application optionally along with the recycling processes of the '093 application provide for an efficient, environmentally friendly method for generating the hydrogen needed for the advancement of a hydrogen based economy. There is a need to further extend the range of applicability of the hydrogen producing reactions of the '707 patent and '935 application. Of particular interest is consideration of the range of starting materials that may be used in the reactions and the suitability of commonly available organic substances for use as reactants. Also of interest is the range of viable reaction conditions that are conducive to the formation of hydrogen gas and optimization of reaction conditions with respect to trade-offs that may be present between reaction efficiency, reaction rate and process cost.